1. Field of the Invention
The present invention relates to ground-terminal antennas for communicating with satellites in geostationary orbit. More particularly, it relates to low-cost, electronically steerable antennas adapted to compensate for motion of a satellite with respect to its fiducial geostationary position, and to electronically steerable multi-beam antennas adapted to compensate for motions of multiple satellites simultaneously.
2. Description of Related Art
Satellites in geostationary orbit are widely used for communications and broadcast applications. When the orbit of a satellite lies along a path 35,786 km directly over the equator, its orbital velocity exactly matches the rate of rotation of the Earth, and the satellite remains fixed in the sky relative to an observer on the ground. This greatly simplifies the design of ground terminals because they can be designed to point in a single fixed direction and do not require bulky motorized gimbals or tracking hardware. However, while a satellite in geostationary orbit should theoretically remain at a fixed location in the sky, perturbations to its orbit caused by interactions of the Sun and Moon as well as the non-spherical shape of the Earth itself cause the orbit of the satellite to drift away from its fiducial geostationary point. As shown in FIGS. 1A-C, a satellite that drifts into a slightly inclined orbit with respect to the equator begins to trace out an elongated figure-eight pattern oriented in the north-south direction in the sky, as seen by the observer on the ground. This motion can result in severe loss of signal by a ground terminal with a simple fixed antenna. A number of methods to address this problem have been developed, but all have significant drawbacks.
One method of addressing this problem is to articulate the ground terminal by adding gimbals and a mechanical tracking system to allow the antenna pointing to be continually adjusted in order to track the satellite. However, such a solution adds significant cost, bulk, and complexity and is not suitable for applications requiring a large number of ground stations, such as direct-broadcast television.
Another method is to selectively broaden the antenna pattern in the north-south direction to account for the increased satellite motion in this direction. For example, a typical one-meter-diameter parabolic antenna operating at Ku band will exhibit a beam width of approximately two degrees. If the antenna reflector is compressed into an ellipse, the pattern in the north-south direction can be stretched to twelve-to-fourteen degrees, covering excursions of a satellite in an orbit inclined up to six or seven degrees with respect to the equator. However, stretching the radiation pattern significantly reduces antenna gain, negatively impacting receive performance and requiring additional power for transmit.
Another method is to actively control the position of the satellite by firing thrusters to perform “station-keeping” maneuvers in order to keep the satellite as close as possible to the equator to minimize north-south excursions. The tighter the station-keeping requirements imposed by the capabilities of the ground terminals, the more frequent are the required station-keeping maneuvers. When the satellite runs out of fuel, it can no longer be maintained in geostationary orbit, so the frequency of such maneuvers directly affects the useful life of the satellite.
Thus, it would be useful to provide a design for a low-cost, compact, ground terminal that does not require mechanical tracking and that would enable a relaxation of tight station-keeping requirements for geostationary satellites in order to reduce fuel consumption and prolong their useful lifetimes.